The present invention relates to a noise reduction system.
It has heretofore been known that, in order to improve the S/N (signal-to-noise) ratio of certain specified transmission systems or specified recording/playback systems, a noise reduction system including a signal compressor and a signal expander is used for the system.
In particular, a noise reduction system wherein the circuit constituent parts of a signal compressor and those of a signal expander are shared and wherein the function of the signal compressor and that of the signal expander can be changed-over by transferring a mode switch has been proposed in the "Journal of the Society of Electronic and Radio Technicians", Vol. 8, May/June 1974. The switchable signal compressor/signal expander of this type is well known in those in the art as the "Dolby B-type Noise Reduction System" (the word "Dolby" is a registered trademark of Dolbey Laboratories).
By changing-over the Dolby B-type noise reduction system to the signal compressor, this system becomes an encoder. The signal compressor (encoder) compresses the dynamic range of an input signal before this input signal is recorded on a recording tape. By changing-over the system to the signal expander, this system becomes a decoder. The signal expander (decoder) restores the linearity of the dynamic range for the input signal. The amount of noise is introduced in a recording/playback process is considerably reduced by this arrangement. Accordingly, the signal compressor/signal expander combination operates as a noise reduction system.
In the Dolby B-type noise reduction system, the operation of signal compression/signal expansion is usually performed for signal components whose frequencies are higher than the frequency value of 200 Hz.
The Dolby C-type noise reduction system has recently been developed on the basis of the Dolby B-type noise reduction system. Although the Dolby C-type system has a similar circuit arrangement to the B-type system, it differs greatly in its noise reduction effect. In particular, whereas the B-type exhibits a noise reduction level of approximately 10 dB at a frequency of 5 kHz, the C-type is improved to a noise reduction level of approximately 20 dB at the frequency of 5 kHz.
FIG. 1 shows circuit blocks for the well-known Dolby C-type noise reduction system when connected to operate as an encoder.
A recording input signal at an input terminal T.sub.1 is applied to the input terminal of a spectral skewing network 11. In order to prevent a high-frequency gain from decreasing during a large-amplitude recording operation due to the characteristic of a tape, signal levels at frequencies of 10 kHz-20 kHz are reduced by the spectral skewing network 11. Thus, encode and decode errors at the specified frequencies of 10 kHz-20 kHz are remarkably reduced.
An output signal from the spectral skewing network 11 is applied to one input terminal of a combining network 12 and is also applied to the other input terminal of the combining network 12 through a high-level side chain 13, whereby an output signal from the combining circuit 12 is provided from a terminal T.sub.2.
Thus, the signal path between the terminals T.sub.1 and T.sub.2 constructs the first level processing circuit of the Dolby C-type encoder. Further, a signal path extending between terminals T.sub.3 and T.sub.4 constructs the second level processing circuit of the Dolby C-type encoder.
When the terminals T.sub.2 and T.sub.3 are connected, the output signal of the combining circuit 12 is applied to an anti-saturation network 14 and a low-level side chain 15. The anti-saturation network 14 operates at a high signal level, thereby to prevent the saturation of the tape, high-frequency signal loss and an increase of the distortion factor.
Since an output signal from the anti-saturation network 14 and an output signal from the low-level side chain 15 are respectively applied to one input terminal and the other input terminal of a combining network 16, the encoded signal of the Dolby C-type encoder can be derived from the output terminal T.sub.4 of the combining network 16.
The encoded output signal of the Dolby B-type encoder typically has amplitude-frequency characteristics such as those shown in FIG. 3. On the other hand, the output signal of the Dolby C-type encoder typically has amplitude-frequency characteristics such as those shown in FIG. 4. In comparing these two figures, it can be seen that as the signal amplitude level lowers, the amplitude value of the frequency component higher than 200 Hz contained in the encoded output signal of the Dolby C-type encoder becomes equal to about double that of the same frequency component contained in the encoded output signal of the Dolby B-type encoder.
FIG. 2 shows circuit blocks for the well-known Dolby C-type noise reduction system connected to operate as a decoder.
An input terminal T.sub.5 has a playback input signal from a playback pre-amplifier applied thereto, and is connected to one input terminal of the combining network 16. An output signal from the combining network 16 is applied to the anti-saturation network 14 through a signal inverter 17. An output signal from the anti-saturation network 14 is supplied to a terminal T.sub.6, and is also supplied to the other input terminal of the combining network 16 through the low-level side chain 15.
Thus, the signal path between the terminals T.sub.5 and T.sub.6 constructs the first level processing circuit of the Dolby C-type decoder. Since the combination of the signal inverter 17 and the combining network 16 executes the subtraction of the signals, signal components higher than 200 Hz in the amplitude-frequency characteristics of the output signal of the first level processing circuit come to have a smaller amplitude value gradually with the lowering of the signal level.
Further, a signal path extending between terminals T.sub.7 and T.sub.8 constructs the second level processing circuit of the Dolby C-type decoder. More specifically, when the terminals T.sub.6 and T.sub.7 are connected, the output signal of the anti-saturation network 14 is supplied to one input terminal of the combining network 12. An output signal from the combining network 12 is supplied to the input terminal of the spectral skewing network 11 through a signal inverter 18, and is further supplied to the other input terminal of the combining network 12 through the high-level side chain 13. Since the combination of the signal inverter 18 and the combining network 12 similarly executes the subtraction of the signals, signal components higher than 200 Hz in the amplitude-frequency characteristics of the output signal of the spectral skewing network 11 to be derived from the terminal T.sub.8 come to have a smaller amplitude value gradually with the lowering of the signal level.
Thus, the overall characteristics of the signal path from the terminal T.sub.5 to the terminal T.sub.8 become inverse to the amplitude-frequency characteristics of FIG. 4.
FIG. 5 illustrates the noise reduction level owing to the noise reduction system based on the combination of the foregoing Dolby C-type encoder and Dolby C-type decoder, and the noise reduction level owing to the Dolby B-type noise reduction system.
Although Dolby B-type and C-type systems certainly do provide excellent noise reduction, some problems do exist. In particular, it is common practice that an audio signal is recorded onto a magnetic tape by superposing an A.C. bias signal of 60 kHz-100 kHz thereon. In this regard, the side chain of the known Dolby noise reduction system of the B-type or the C-type not only reponds to audio signal components of high frequencies, but also responds sensitively to the A.C. bias signal of 60 kHz-100 kHz. Therefore, the known noise reduction Dolby B-type and C-type systems involved comparatively large encode and decode errors.